1. Field of the Invention
The invention relates generally to the production of bio-oils from aquatic biomass. More particularly the invention relates to a process that can be carried out at a relatively low temperature and produces fresh water as a valuable by-product.
2. Description of the Related Art
Aquatic biomass has been recognized as a potential source of fuel. Methods for converting aquatic biomass generally belong to one of two classes: biochemical methods, such as fermentation, and thermochemical methods. The latter include direct combustion, heat decomposition, gasification, and liquefaction.
By its nature aquatic biomass contains considerable quantities of water. Some of the energy conversion processes require the removal of (most of) the water. An example is direct combustion, which requires that the aquatic biomass first be dewatered and dried, prior to its use as a combustion fuel. A drying step significantly adds to the cost and complexity of the process. Therefore, processes have been proposed that do not require the aquatic biomass to be dried.
WO 2007/101172 discloses a process for the production of ethanol from algae. The process requires starch producing algae. The process comprises a mechanical crushing step in which algae cells are crushed to make the starch accessible prior to the addition of fermentation yeast. Ethanol produced in the fermentation step is separated from the water by, for example, distillation.
Although the process disclosed in WO 2007/101172 does not require the removal of water from the aquatic biomass prior to the fermentation, it involves the difficult separation of ethanol from water. In addition, yeast fermentation puts constraints on the composition of the water feed in terms of salt content.
Dote et al., “Recovery of liquid fuel from hydrocarbon-rich microalgae by thermochemical liquefaction”, Fuel 1994, pp 1855-1857, discloses a process for the liquefaction of Botryococcus braunii. The process is carried out in aqueous medium in an autoclave under a nitrogen atmosphere. Conversion experiments were carried out at 200, 300, and 340° C. Hydrocarbons produced in the process were separated from the water phase by hexane extraction. The use of nitrogen, the high pressure, and the hexane extraction step make this process uneconomical for industrial scale conversion of aquatic biomass.
Yang et al., “Analysis of energy conversion characteristics in liquefaction of algae. Resources”. Conservation and Recycling 43 (2004) 21-33, disclose a process for the liquefaction of Microcystis viridis. The reaction is carried out in an autoclave at 300 or 340° C. The pressure of the autoclave was 10-20 MPa in order to decrease the water evaporation. The reaction mixture was extracted with chloroform.
Thus, there is a particular need for a process for converting aquatic biomass to bio-oil that does not require the pre-drying of the biomass. There is a further need for such a process that can be carried out at atmospheric or near-atmospheric pressures. There is a further need for a process that does not require an expensive separation step for removing water from the reaction product. Ideally, water is recovered in a form that allows its use in agriculture or for human consumption.